(1) Field of Invention
The present invention relates generally to sensing output states of data lines of addressable memory circuits. More specifically, the preferred embodiment of the present invention is related to adjusting precharge voltages of data lines to substantially equal threshold voltages of sense amplifiers used to detect voltage changes on the data lines.
(2) Description of the Prior Art
Read Only Memories (ROMs) typically have data lines coupled to addressable memory cells and provide a data signal on a data line having a first or second amplitude corresponding to a logical "one" or "zero" associated with a selected data bit. A sense amplifier is coupled to each data line for sensing small changes in the amplitude of the data signal and for providing a logic level output signal on an output line in response to the value of the data signal. The sense amplifier is characterized by a threshold voltage having an amplitude between the first and second amplitudes and provides the logic level output signal representative of the amplitude of the data signal relative to the threshold voltage.
One of the limitations of conventional memory circuits is related to the large capacitance of the data lines. Especially in the case of large ROM arrays, the capacitance of the data lines is large relative to the size of the small array transistors of the addressable memory cells. This capacitance limits the speed at which the array transistors can charge or discharge the data lines, and thus limits the ultimate speed at which data can be read from the ROM.
Conventional memory circuits operate in two modes corresponding to alternating first and second clock phases. In a precharge mode, corresponding to the first clock phase, the data lines are charged to a supply voltage. In a sense mode, corresponding to a subsequent second clock phase, the data lines are selectively discharged to a ground voltage by the array transistors. Thus, the minimum time duration of the second phase is determined by the time needed for the data lines to be discharged from the supply voltage to less than the threshold voltage of the sense amplifier.